1. Field of the Invention
The present invention relates to a voltage controlled oscillator.
2. Description of the Related Art
A voltage controlled oscillator is disclosed in Japanese Patent Application Laid-Open No. 2004-320664. Japanese Patent Application Laid-Open No. 2004-320664 discloses that a variable frequency amplitude is enlarged without degradation of phase noise characteristics, by including: a varactor diode circuit in which a plurality of circuits having a varactor diode and an open stub connected in series are connected in parallel; and a plurality of control circuits for independently applying a control voltage to each varactor diode in the varactor diode circuit, and by variably controlling control voltages independently applied to each varactor diode to one of them in turn in a range from their lower limit to upper limit, in order to control compound capacitance of the varactor diode circuit in the entire variable range thereof.
Because of recent adoption of a multivalued modulation system involved in higher-capacitance radio communication system and the like, requirements for the signal source of the communication signal have become stricter. In particular, because quality of communication is higher, phase noise characteristics of a local oscillator (LO) signal which is the signal source have become a very important element.
In radio communication, as the signal source, a voltage controlled oscillator (hereinafter, called “VCO”) controlled with PLL (Phase Locked Loop) is generally used. FIG. 1 shows the waveform spectrum of an LO signal outputted from a local oscillator used in a PLL circuit constituting a radio system. The PLL circuit has a general configuration which includes the local oscillator, a phase comparator, a loop filter, a variable frequency divider and a voltage controlled oscillator controlled with PLL.
Among the waveforms shown in FIG. 1, a waveform in area 51 is compressed by PLL, and its value and range are determined by characteristics of the loop filter constituting the PLL circuit.
Area 52 is where the 1/f noise of a semiconductor becomes dominant, and area 53 is where a Q value (quality factor: a parameter relating to a loss of a resonant circuit, as the loss becomes less, the Q value becomes larger) of a circuit including a resonator starts to become dominant, and it is necessary to design a circuit configuration of the signal source so as to improve this.
As for a variable element used as a basis for controlling the frequency in the VCO, change in capacitance of a varactor diode is often used. Such varactor diode often used in an oscillator in a microwave band is as follows:    1. GaAs of HyperAbrupt Type    2. Si of Abrupt TypeThe most suitable diode that is used is selected based upon its operational physical characteristics.
A GaAs varactor diode of HyperAbrupt type has a large capacitance change ratio and a small series resistance Rs. As a result, when is used in an oscillation circuit, a higher Q value can be provided. However, because of large change in impurity concentration on a joint surface, a lattice defect or an interface state is likely to occur, resulting in larger 1/f noise.
On the one hand, because a Si varactor diode of the Abrupt type has a small capacitance change ratio and a small series resistance Rs, when it is used in the oscillation circuit, a Q value tends to be smaller. However, compared to the HyperAbrupt type, because of a smaller change in impurity concentration on a joint surface, the 1/f noise is lower.
The GaAs varactor diode of HyperAbrupt type has a larger capacitance change ratio and better linear capacitance change during application of the voltage which therefore makes it easier to design for use in an oscillation circuit. However, degradation of a phase noise is likely to occur due to the effect caused by 1/f noise.
Further, when a Si varactor diode of the Abrupt type is selected to improve 1/f noise, degradation of phase noise characteristics may be caused due to a decrease in the variable frequency range caused by the small capacitance change ratio, and the low Q value.
It is difficult to improve by circuit design, because the influence of device characteristics dominates all these problems. Moreover, it is very hard to conduct a trade-off, which makes the circuit design including device selection complicated.
Concerning the element of a resonator in the voltage controlled oscillator, there may be a corner frequency fc. The corner frequency fc is a frequency which is an inflection point of phase noise characteristics, and in a frequency range that is not higher than the corner frequency fc, the effect of 1/f noise becomes a dominant factor for determining phase noise characteristics. In a frequency range that is not lower than the corner frequency fc, the Q value of the resonator circuit becomes a dominant factor for determining the phase noise characteristics.
A VCO using a varactor diode having a wide, variable frequency range has its phase noise characteristics determined by characteristics of the varactor diode which is the variable element. In order to enlarge the variable frequency range, a GaAs varactor diode of the HyperAbrupt type having a large capacitance change ratio is used. In this case, a problem arises in which degradation of phase noise characteristics becomes significant, due to the effect caused by the 1/f noise of the varactor diode, in a range that is not higher than a corner frequency fc.